Kinetics and Mechanism of the Formation of (1,5)bis(2-hydroxybenzamido)3- azapentaneiron(III) and its Reactions with Thiocyanate, Azide, Acetate, Sulfur(IV) and Ascorbic Acid in Solution, and the Synthesis and Characterization of (nitrato)bis- (2-hydroxybenzamido)3-azapentaneiron(III). The Role of Phenol–amide–amine Coordination

Abstract

The complexation of iron(III) with (1,5)bis(2-hydroxybenzamido)3-azapentane (H 2 L) under varying [H + ] T (0.01-0.1 mol dm )3 ) and [Fe III ] T (3.0 · 10 )4 -1.7 · 10 )2 , ½L T ¼ ð0:5 À 1:0Þ Â 10 À4 mol dm )3 ) (I=0.3 mol dm )3 , 10% v/v, MeOH + H 2 O, 25.0°C) was reversible and displayed monophasic kinetics; the dominant path involved FeOH 2+ and H 3 L + . The mechanism is essentially a dissociative interchange (I d ) and dissociation of the aqua ligand from the encounter complex, [Fe(OH 2 ) 5 OH 2+ , H 3 L + ] is rate-limiting. Equilibrium measurements indicated that the ligand binds iron(III) in a bidentate, tetradentate and pentadentate fashion under varying pH conditions. Iron(III) promoted deprotonation of the phenol moieties, and sec-NH þ 2 of the dien unit are in tune with this proposition. The octahedral coordination of ½FeðHL=LÞðOH 2 Þ 2þ=þ is further supported by the aqua ligand substitution by AcO ) , NCS ) , N À 3 =N 3 H; SO 2À 3 =HSO À 3 . However, marked pK perturbation of the bound ascorbate in [Fe(L)(HAsc/Asc)] 0/) (DpK {[Fe(L)(HAsc)] ) HAsc)} =6) is compelling evidence for chelation of HAsc ) / Asc 2) leading to unusual hepta coordination of iron(III) in the ascorbate complexes. Despite the multidentate nature of the ligand, its iron(III) complexes remain sensitive to reduction by S IV and ascorbic acid.The complex (nitrato){(1,5)bis(2-hydroxybenzamido)3-azapentane}iron(III) has been synthesised and characterised by elemental analysis, i.r. and u.v.-vis spectral measurements. The room temperature magnetic moment (l eff =4.2 BM) conforms to the intermediate spin state of iron(III) (S=3/2) which is further supported by e.s.r. measurements (77 K, g=4.2, 8.1) and the 57 Fe Mo¨ssbauer spectrum (d=0.41 mm s )1 ; D E Q ¼ 0:78 mm/s). The cyclic voltametry (MeOH, TEAP as background electrolyte) display only one quasi-reversible peak in the )0.254 to )0.4 V range (vs. SCE), the irreversibility being due to the formation of an iron(II) complex which dissociates under the experimental conditions.

Kinetics and mechanism of the formation of (1,8)bis(2-hydroxybenzamido)3,6-diazaoctaneiron(III) and its reactions with thiocyanate, azide, acetate, sulfur(IV) and ascorbic acid in solution, and the synthesis and characterization of a novel oxo bridged diiron(III) complex. The role of phenol–amide–amine coordination

“…We propose structure (A) for the species [Fe(H 3 L)] 4+ consistent with its u.v.-vis spectrum {k max , nm (10 )3 e, dm 3 mol )1 cm )1 ): 520 (2.00 ± 0.11) for the phenolate-Fe III LMCT band}. The observed reduced reactivity of Fe(OH 2 ) 5 OH 2+ (see values of k 1 /k 2 in Table 3) is reconciled with the attenuation of labilizing action of the bound hydroxo group due to hydrogen bonding with the protonated ligands as discussed in our earlier work [12], H 4 L 2+ being more efficient than H 3 L¢ + in this respect. …”

“…However, the reaction of Fe(OH 2 ) 6 3+ with H 4 L 2+ leading to the same product was also detectable kinetically despite relatively stronger coulombic repulsion between the two species and the established associative interchange mechanism (I a ) for the ligand substitution of the hexa-aqua cation unlike for its aqua-hydroxo analogue, [Fe(OH 2 ) 5 OH] 2+ (I d mechanism) [25,26]. Similar observation was also made for the protonated form of 1,5-bis(2-hydroxybenzamido)3-azapentane (H 3 L¢ + ) [12]. In the case of 1,3-bis(2-hydroxybenzamido)propane (H 2 L¢¢) and 1,2-bis(2-hydroxy benzamido)ethane (H 2 L¢¢¢), the neutral form of the ligands were the reacting species.…”

“…This is in keeping with the fast association equilibrium of the iron(III) species and the participating ligands followed by the slow interchange of the ligand with the coordinated H 2 O molecule in the complexes, [Fe(OH 2 ) 6 3+ , H 2+n L n + ] or [Fe(OH 2 ) 5 OH 2+ , H 2+n L n +] (n = 0-2). The protonation of the sec-NH of L and L¢ does not significantly reduce their abilities for outer sphere association with the (aqua)iron(III) species, and the rate limiting process involves interchange of the aqua ligand by the phenol moiety [12] followed by the rapid chelation. We propose structure (A) for the species [Fe(H 3 L)] 4+ consistent with its u.v.-vis spectrum {k max , nm (10 )3 e, dm 3 mol )1 cm )1 ): 520 (2.00 ± 0.11) for the phenolate-Fe III LMCT band}.…”

“…The pH data (i.e. meter readings) at pH <7 were converted to ) log[H + ] by a calibration curve as described earlier [12]. At pH >7 (MeOH + H 2 O media) no such calibration was attempted and the measured pH values after a minor solvent correction [16] were used for all calculations.…”

“…This work stems from our general interest in the reactions of sulfur(IV) with redox active transition metal complexes [10] as well as current research activities on the carboxamido and related complexes of iron(III) as models of several hydrolytic and redox enzymes [1, 5,11]. In a preceding paper [12] we reported the structural and kinetic investigation of a mononuclear iron(III) complex of a pentadentate phenol-amide-amine ligand, (1,5)bis(2-hydroxybenzamido)3-azapentane, which prompted us to undertake the present work using the linear hexadentate ligand, (1,8)bis(2-hydroxybenzamido) 3,6-diazaoctane (H 2 L, I). In the chosen ligand, the trien (triethylenetetramine) skeleton acts as a central organizational spacer unit which connects two terminal bidentate salicylamide moieties as the pendant arms offering multidentate coordination ranging from two to six.…”